fix crash if trigger isn't set

The GUI defaulted to 'lock-screen', but the CLI did not. And it was crashing if 'trigger' wasn't defined in the config file when executed over the CLI. This fixes it

 * #14 (comment)

	INFO: using DATA_DIR:|/home/user/.local/share/.buskill|
	DEBUG: CONF_FILE:|/home/user/.local/share/.buskill/config.ini|

	Traceback (most recent call last):
  	File "/tmp/.mount_buskilReWwl8/opt/python3.7/lib/python3.7/configparser.py", line 788, in get
    	value = d[option]
  	File "/tmp/.mount_buskilReWwl8/opt/python3.7/lib/python3.7/collections/__init__.py", line 916, in __getitem__
    	return self.__missing__(key)            # support subclasses that define __missing__
  	File "/tmp/.mount_buskilReWwl8/opt/python3.7/lib/python3.7/collections/__init__.py", line 908, in __missing__
    	raise KeyError(key)
	KeyError: 'trigger'

	During handling of the above exception, another exception occurred:

	Traceback (most recent call last):
  	File "/tmp/.mount_buskilReWwl8/opt/src/main.py", line 129, in <module>
    	bk = packages.buskill.BusKill()
  	File "/tmp/.mount_buskilReWwl8/opt/src/packages/buskill/__init__.py", line 426, in __init__
    	self.set_trigger( self.config.get('buskill', 'trigger') )
  	File "/tmp/.mount_buskilReWwl8/opt/python3.7/lib/python3.7/configparser.py", line 791, in get
    	raise NoOptionError(option, section)
	configparser.NoOptionError: No option 'trigger' in section: 'buskill'
	user@disp6142:~/buskill-lin--x86_64$
	user@disp6142:~/buskill-lin--x86_64$ ./buskill-.AppImage --arm

src/packages/buskill/__init__.py

@@ -423,7 +423,11 @@ def __init__(self):
 		# set the default trigger to what's defined in the config file
 		self.config = configparser.ConfigParser()
 		self.config.read( self.CONF_FILE )
-		self.set_trigger( self.config.get('buskill', 'trigger') )
+		if self.config.has_option('buskill', 'trigger'):
+			trigger = self.config.get('buskill', 'trigger')
+		else:
+			trigger = 'lock-screen'
+		self.set_trigger( trigger )
 
 		# handle conditions where this version was already upgraded by a newer
 		# version or if this is a version that upgraded an older version